Re: [RFC PATCH 0/2] remove SWAP_MAP_SHMEM

[Baolin Wang <baolin.wang@linux.alibaba.com>]

On 2024/9/26 09:59, Huang, Ying wrote:
> Nhat Pham <nphamcs@gmail.com> writes:
> 
>> On Tue, Sep 24, 2024 at 6:53 PM Baolin Wang
>> <baolin.wang@linux.alibaba.com> wrote:
>>>
>>>
>>> One benefit I can mention is that removing 'SWAP_MAP_SHMEM' can help to
>>> batch free shmem swap entries in __swap_entries_free(), similar to the
>>> commit bea67dcc5eea ("mm: attempt to batch free swap entries for
>>> zap_pte_range()") did, which can improve the performance of shmem mTHP
>>> munmap() function based on my testing.
>>
>> Yeah, the problem with having an extraneous state is you have to
>> constantly check for it in code, and/or keep it in mind when you
>> develop things. I've been constantly having to check for this state
>> when I develop code around this area, and it gets old fast.
>>
>> If we can use it to optimize something, I can understand keeping it.
>> But it just seems like dead code to me :)
>>
>> My preference is to do this as simply as possible - add another case
>> (usage == 1, nr > 1, and we need to add swap continuations) in the
>> check in __swap_duplicate()'s first loop, and just WARN right there.
>>
>> That case CANNOT happen UNLESS we introduce a bug, or have a new use
>> case. When we actually have a use case, we can always introduce
>> handling/fallback logic for that case.
>>
>> Barry, Yosry, Baolin, Ying, how do you feel about this?
> 
> Sounds good to me to just WARN now.  We can always improve when it's
> necessary.

+1. Agreed.